Method for producing forged material by using large vertical continuous casting round billet, product and application thereof

Abstract

The invention discloses a method for producing a forging material by using a large vertical continuous casting round billet, a product and application of the product, and relates to the technical field of metal hot working.The method for producing the forging material by using the large vertical continuous casting round billet comprises the following steps that firstly, burdening, EBT electric furnace steelmaking, LF furnace refining, VD vacuum degassing, phi 500-1200 mm large vertical round billet continuous casting and electroslag feeding are sequentially conducted, a vertical continuous casting round billet is obtained; wherein the refining temperature of the LF furnace is 1560-1590 DEG C, heat preservation is conducted for 40-60 min, and [O] in molten steel is controlled to be smaller than or equal to 20 ppm by feeding an aluminum wire of 0.1-0.2 kg / t steel for deoxidation; according to the method, the internal stress of the finished forging material is smaller than or equal to 80 MPa and is reduced by 27% or above compared with that of a traditional process by relying on the heat delivery technology of a nanometer heat insulation cover and electric heat tracing in cooperation with a customized annealing curve and ultrasonic stress monitoring every 2 h, and the problem of internal stress accumulation caused by excessive forging or multi-heating-number machining is solved.

Description

Technical Field

[0001] This invention relates to the field of metal hot working technology, specifically to a method, product, and application of producing forgings using large vertical continuous casting round billets. Background Technology

[0002] In strategic emerging industries such as wind power, nuclear power, and high-end marine engineering, large-section circular forgings are key basic raw materials for manufacturing core components such as wind turbine main shafts, nuclear power primary loop pipelines, and heavy pressure-bearing components. These components place stringent requirements on the performance of forgings: they must possess high purity to avoid hydrogen-induced cracking and fatigue failure, low internal stress to prevent deformation during subsequent processing, and a fine-grained, uniform microstructure to ensure the stability of mechanical properties.

[0003] However, the current production process of large forgings in the industry still faces a core technological bottleneck: it is difficult to achieve a balance between high purity, low internal stress, and fine grain structure in the production of large-diameter, long-length forgings, resulting in the forging performance failing to meet the requirements of high-end equipment. Specifically, this manifests as follows:

[0004] Existing processes are mainly divided into two categories: the first is the traditional process of die casting and forging. During die casting, the solidification rate of molten steel is uneven, which easily leads to shrinkage cavities, central porosity, and compositional segregation. Moreover, the molten steel is only subjected to simple LF refining treatment, and the [H] content is generally ≥3ppm, [N] ≥100ppm, and the particle size of hard and brittle inclusions such as Al2O3 is often ≥10μm. These defects need to be compensated by forging with a high reduction rate, but excessive forging will cause the internal stress of the forging material to rise to more than 120MPa. At the same time, the as-cast structure is coarse, and it is impossible to balance purity and structural uniformity. The second is the improved process of small-size continuous casting billets and multi-fire forging. Although it can reduce some die casting defects, the production of large-diameter forgings from small-size billets requires more than 3 fire forgings. Multiple heating and cooling not only lead to abnormal grain growth, but also accumulate internal stress. Moreover, there is a lack of targeted inclusion modification and gas removal methods, and the purity of molten steel still cannot meet the high-end requirements of nuclear power and wind power.

[0005] The aforementioned technical bottlenecks directly result in the following: large-size forgings produced by traditional processes have a fatigue strength of only 400-450MPa, and the salt spray life of 316L stainless steel forgings is less than 800h. At the same time, downstream processing causes rapid mold wear and a scratch rate of ≥8% on the inner surface of the pipes, which restricts the upgrading of the high-end equipment manufacturing industry.

[0006] Therefore, we propose a method, products, and applications for producing forgings using large vertical continuous casting round billets, in order to solve the problems mentioned above. Summary of the Invention

[0007] The purpose of this invention is to provide a method, product and application for producing forgings using large vertical continuous casting round billets, in order to solve the problems mentioned in the background art that it is difficult to achieve high purity, low internal stress and fine grain structure in the current production of large round cross section forgings on the market, and that the downstream processing adaptability is poor, the production energy consumption is high, the whole process quality traceability is lacking, and it cannot meet the needs of high-end fields such as wind power and nuclear power.

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] A method for producing forgings using large vertical continuous casting round billets includes the following steps:

[0010] Step 1: The process involves batching, EBT electric furnace steelmaking, LF furnace refining, VD vacuum degassing, continuous casting of φ500-1200mm large vertical round billets, and electroslag feeding to obtain vertical continuous casting round billets.

[0011] The refining temperature in the LF furnace is 1560-1590℃, and the holding time is 40-60 minutes. Deoxidation is achieved by feeding 0.1-0.2 kg / t of aluminum wire into the steel, and the [O] in the molten steel is controlled to be ≤20ppm.

[0012] In the continuous casting process of large vertical round billets, a multi-field coordinated control method is adopted, which includes electromagnetic stirring in the crystallizer, electromagnetic braking in the secondary cooling zone, and alternating electromagnetic stirring at the end. Among them, the M-EMS current is 150-300A and the frequency is 2-5Hz, the EMBr braking intensity is 0.1-0.3T, the F-EMS current is 180-250A, the frequency is 3-4Hz, and the alternation period is 20-40s. The secondary cooling zone adopts zoned water mist cooling, with the cooling water volume of the 1st to 3rd stages being 0.6-0.9L / kg steel and the cooling water volume of the 4th to 6th stages being 0.4-0.6L / kg steel, and the temperature difference between the inside and outside of the billet is controlled to be ≤50℃.

[0013] Electroslag feeding uses pre-melted slag with a composition of 250-55% CaF2, 25-30% Al2O3, and 15-20% CaO, with an arc power of 500-700kW, feeding until the billet is completely solidified;

[0014] During billet preparation, after refining in the LF furnace and before vacuum degassing in the VD furnace, the circulation time is 15-25 minutes. After treatment, the molten steel has [H] ≤ 2ppm and [N] ≤ 80ppm.

[0015] In the calcium treatment process for modifying inclusions, calcium wire with a calcium content ≥98% is fed in at a feeding speed of 2-3 m / min and a feeding amount of 0.2-0.3 kg / t steel. This process converts the hard and brittle Al2O3 inclusions in the molten steel into low-melting-point calcium aluminates, while controlling the inclusion particle size to ≤5 μm.

[0016] Step 2: The vertical continuous casting round billet obtained in Step 1 is heat-transferred through a nano-insulation cover and an electric heat tracing compensation insulation device. The nano-insulation cover is made of SiO2 aerogel material with a thermal conductivity ≤0.02W / (m・K) and the electric heat tracing power is 5-10kW.

[0017] Among them, by arranging 3-5 thermocouples on the surface of the billet to monitor the temperature in real time, and starting electric heating when the temperature drop is >50℃, the temperature of the middle surface of the billet is 870-930℃ when it enters the furnace, and the temperature drop fluctuation is ≤30℃.

[0018] Step 3: The hot-delivered vertical continuous casting round billet is fed into a segmented regenerative combustion heating furnace, and a heating curve is automatically generated according to the steel grade and diameter of the billet.

[0019] For alloy structural steel φ1000mm casting billet, the heating curve is from room temperature to 600℃ at 80℃ / h and hold for 2h, to 1000℃ at 60℃ / h and hold for 3h, and then to 1220℃ at 50℃ / h and hold for 5h.

[0020] Step 4: The heated vertical continuous casting billet is sent to a 1600T or 3150T forging press for multi-pass gradient reduction forging. The total number of reductions is 3-5 passes, with the first pass having a reduction rate of 20-25%, the intermediate passes having a reduction rate of 15-20%, and the last pass having a reduction rate of 10-15%.

[0021] During the forging process, an infrared thermal imager is used to monitor the surface temperature of the forging throughout the process. When the temperature is lower than the lower limit of the forging temperature, the pressing speed is reduced from 5 mm / s to 2 mm / s. Finally, forgings with a diameter of φ250-1000 mm and a length of L≤12000 mm are produced.

[0022] Step 5: Customize the annealing curve according to the steel grade and size of the forging. During the annealing process, use an ultrasonic stress meter to detect the internal stress of the forging every 2 hours. When the internal stress is >100MPa, reduce the cooling rate by 10℃ / h to ensure that the internal stress of the finished forging is ≤80MPa.

[0023] In step five, for 20Cr2Ni4 steel φ550mm forging, the annealing curve is as follows: hold at 650℃ for 4h → cool down to 300℃ at 40℃ / h and hold for 16h → heat up to 890℃ at 60℃ / h and hold for 8h → air cool to 400℃ → furnace cool to 300℃ → heat up to 650℃ at 70℃ / h and hold for 35h → cool down to 400℃ at 40℃ / h → cool down to 150℃ at 50℃ / h.

[0024] In step one, the large vertical round billet continuous casting is a single-strand or two-strand continuous casting, and each billet has a length L≤8000mm and a weight ≤70t; in step four, the forging produced corresponds to alloy structural steel, tool steel or stainless steel.

[0025] Step 6: After annealing, the forging is machined, peeled, locally ground, and sawed in sequence; the machining allowance for peeling is 3-5mm, the surface roughness Ra of the forging after local grinding is ≤1.6μm, and the length deviation of the forging after sawing is ±2mm.

[0026] Step 7: Conduct visual inspection, dimensional accuracy inspection, non-destructive testing, microstructure inspection, and mechanical property inspection;

[0027] Among them, the appearance inspection requires that the forging material be free of cracks and inclusions; the dimensional accuracy inspection is carried out by laser diameter gauge; in the non-destructive testing, UT conforms to EN10228-3 class2 standard, and MT conforms to JB / T4730.4 standard; the microstructure inspection is carried out by metallographic microscope, and the grain grade of the forging material is required to be 5-8; the mechanical property inspection includes tensile, impact and hardness tests.

[0028] Alloy structural steel forgings have a tensile strength ≥800MPa, a yield strength ≥600MPa, and an elongation ≥15%; 316L stainless steel forgings have a tensile strength ≥515MPa, a yield strength ≥205MPa, and an elongation ≥40%.

[0029] Step 8: Perform customized pretreatment based on the downstream processing type of the forging material;

[0030] When used for precision forging of bars, the surface is sequentially shot-peened and phosphated. The shot peening pellet diameter is 0.5-1 mm, the shot peening intensity is 0.2-0.3 mmA, and the phosphate film thickness after phosphate treatment is 5-10 μm. When used for rolling of wire rods, the surface is sequentially descaled by high-pressure water jet and aged at low temperature. The high-pressure water jet pressure is 20-30 MPa, the low-temperature aging temperature is 500℃, and the holding time is 2 h. When used for piercing of pipes, the inner surface is sequentially subjected to eddy current testing and coated with glass lubricant. The eddy current testing frequency is 50-100 kHz, and the glass lubricant thickness is 3-5 μm.

[0031] Compared with the prior art, the beneficial effects of the present invention are:

[0032] This invention employs a three-stage purification process—LF furnace refining, vacuum degassing, and calcium treatment to modify inclusions—to control the [H] content of molten steel to ≤2ppm and [N] to ≤80ppm, reducing it by more than 50% compared to traditional processes. Simultaneously, it transforms hard and brittle Al2O3 inclusions into low-melting-point calcium aluminates with a particle size ≤5μm and an inclusion rating ≤1.0, completely eliminating the initiation source of hydrogen-induced cracking and fatigue failure, thus laying the foundation for high-performance forgings.

[0033] This invention relies on nano-insulation covers and electric heat tracing technology, combined with customized annealing curves and ultrasonic stress monitoring every 2 hours, to ultimately achieve an internal stress of ≤80MPa in the finished forging material, which is more than 27% lower than that of traditional processes, thus solving the problem of internal stress accumulation caused by excessive forging or multiple heat treatments.

[0034] This invention achieves precise grain control throughout the entire process of casting and forging by controlling the temperature difference between the inside and outside of the billet to ≤50℃ through multi-stage continuous casting, thereby reducing the initial coarse grains. Combined with 3-5 passes of gradient forging, the grain grade of the forging material reaches 5-8, which is 1-2 grades finer than the traditional process, and the uniformity of the structure is improved by 40%.

[0035] The above overview is for illustrative purposes only and is not intended to be limiting in any way. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features of the invention will become readily apparent from the accompanying drawings and the following detailed description. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the method for producing forgings using large vertical continuous casting round billets according to the present invention.

[0037] Figure 2 This is a schematic diagram of the entire process of quality control and downstream compatibility preprocessing in this invention. Detailed Implementation

[0038] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0039] Example 1: 20Cr2Ni4 alloy structural steel φ600mm wind turbine main shaft forging material

[0040] From the appendix Figure 1 , 2 As shown, the process steps for producing forgings using large vertical continuous casting round billets are as follows:

[0041] 1. Billet preparation

[0042] Raw material ratio: 78% blast furnace iron, 20% 20Cr2Ni4 recycled scrap steel, and 2% high-carbon ferrochrome. After batching, ensure the target composition of the molten steel meets the following requirements: C 0.17-0.23%, Si 0.17-0.37%, Mn 0.30-0.60%, Cr 1.25-1.65%, Ni 3.25-3.65%, with residual S and P content ≤0.035% and residual Cu content ≤0.030%.

[0043] EBT electric arc furnace steelmaking: An eccentric bottom tapping electric arc furnace is used, and the tapping temperature is controlled at 1640-1660℃ to avoid slag being carried into subsequent refining processes and affecting the purity of molten steel.

[0044] LF furnace refining: The molten steel after tapping from the electric furnace is transferred to the ladle refining furnace. The refining temperature is maintained at 1570-1590℃ and the holding time is 50min. During this period, deoxidation is carried out by feeding in pure aluminum wire. The oxygen content [O] in the molten steel is ultimately controlled to be ≤18ppm, and the composition of the molten steel is adjusted to the target range.

[0045] Vacuum degassing: The refined molten steel is sent to the RH vacuum treatment device, the vacuum degree is controlled at ≤0.9Pa, and the circulation treatment time is 18min. The main purpose is to remove hydrogen and nitrogen gases from the molten steel. After treatment, the molten steel is tested to have [H]≤1.9ppm and [N]≤75ppm, which reduces the risk of hydrogen-induced cracking in subsequent forging materials.

[0046] Calcium treatment for inclusion modification: After RH treatment, solid calcium wire with a calcium content ≥98% is fed into the molten steel at a feeding speed of 2.2 m / min and a feeding amount of 0.22 kg / t steel. Through the reaction of calcium with the hard and brittle Al2O3 inclusions in the molten steel, the inclusions are converted into low-melting-point calcium aluminate inclusions, and the inclusion particle size is ultimately controlled to be ≤4.5 μm.

[0047] VD Vacuum Degassing: After calcium treatment, the molten steel is transferred to the VD vacuum degassing device with a vacuum degree ≤4Pa and a holding time of 12min to further remove residual trace gases and fine inclusions from the molten steel, ensuring that the purity of the molten steel meets the requirements of high-end forgings.

[0048] Large vertical round billet continuous casting: A single-strand vertical continuous casting machine is used, with a billet diameter of φ1200mm, a billet length of 8000mm, and a single billet weight of 68t; the continuous casting process adopts a multi-field coordinated control method, including crystallizer electromagnetic stirring (M-EMS), secondary cooling zone electromagnetic braking (EMBr), and end-of-line alternating electromagnetic stirring (F-EMS).

[0049] M-EMS: 300A current, 2Hz frequency, promotes the flow of molten steel in the crystallizer and reduces component segregation in the initial solidification zone;

[0050] EMBr: Braking strength 0.25T, the braking strength is dynamically adjusted according to the infrared temperature measurement data of the billet surface to suppress the churning of molten steel and reduce slag entrapment;

[0051] F-EMS: Current 250A, frequency 3Hz, alternating stirring cycle 30s, refines the central grains of the billet and reduces central porosity;

[0052] Secondary cooling zone: Zoned water mist cooling is adopted, with 0.85L / kg steel for cooling zones 1-3 and 0.55L / kg steel for cooling zones 4-6. The final temperature difference between the inside and outside of the billet is controlled to ≤42℃ to avoid thermal stress cracks caused by excessive temperature difference in the billet.

[0053] Electroslag feeding: After continuous casting, the tail end of the billet is subjected to electroslag feeding treatment; the pre-melted slag composition is CaF2 52%, Al2O3 28%, CaO 20% by mass fraction, the arc power is 650kW, the feeding time is 45min, until the tail end of the billet is completely solidified, eliminating shrinkage cavities and porosity defects at the tail end of the billet, and ensuring the overall quality of the billet is uniform.

[0054] 2. Hot delivery process

[0055] A composite heat preservation device consisting of a nano-insulation cover and electric heat tracing compensation, wherein the nano-insulation cover is made of SiO2 aerogel material and the electric heat tracing device has a power of 8kW.

[0056] Five thermocouples are evenly arranged along the length of the billet surface to monitor the temperature of the middle surface of the billet in real time. When the billet temperature drops to 870℃, the electric heat tracing compensation is automatically activated to ensure that the temperature of the middle surface of the billet is 870-940℃ when it is sent into the heating furnace, with a temperature drop fluctuation of ≤30℃, thereby reducing energy loss and structural stress during the billet cooling process.

[0057] 3. Heating process

[0058] The heating equipment adopts a segmented regenerative combustion furnace, with 5 heating zones and 12 temperature monitoring points inside the furnace;

[0059] Based on the characteristics of 20Cr2Ni4 steel and the size of the φ1200mm billet, a heating curve is automatically generated: room temperature → heating at 80℃ / h to 600℃ and holding for 2h to eliminate the temperature gradient during the hot delivery of the billet → heating at 60℃ / h to 1000℃ and holding for 3h to avoid billet cracking caused by rapid heating at low temperature → heating at 50℃ / h to 1230℃ and holding for 5.5h to ensure that the core of the billet is fully heated and the temperature is uniform.

[0060] During the heating process, the burner power of each heating zone is adjusted to control the temperature uniformity of the furnace within ±9℃, thus preventing uneven deformation of the billet during forging due to local temperature differences.

[0061] 4. Forging process

[0062] The forging equipment uses a 3150T hydraulic forging press. Following the principles of first eliminating internal defects, then refining the grains, and finally ensuring dimensional accuracy, it employs a four-pass gradient pressing process.

[0063] Pass 1: Forge the φ1200mm billet to φ1050mm with a reduction rate of 24.4% and a forging speed of 4mm / s, focusing on eliminating porosity and shrinkage cavities in the center of the billet;

[0064] Second pass: Forging the φ1050mm billet to φ920mm with a reduction rate of 23.1% and a forging speed of 3.5mm / s to further densify the microstructure;

[0065] Third pass: Forging the φ920mm billet to φ780mm with a reduction rate of 19.3% and a forging speed of 3mm / s to optimize the internal texture of the forging material;

[0066] Pass 4: Forge the φ780mm billet to φ600mm with a reduction rate of 12.8% and a forging speed of 2.5mm / s to ensure the dimensional accuracy and surface quality of the forging material;

[0067] During the forging process, an infrared thermal imager is used to monitor the surface temperature of the forging throughout the process to ensure that the minimum surface temperature of the forging is ≥850℃; when the local temperature is detected to be lower than 850℃, the forging billet is returned to the furnace and heated to the specified temperature for uniform heating.

[0068] 5. Annealing process

[0069] Based on the dimensions and internal stress control requirements of the φ600mm 20Cr2Ni4 forging, a customized annealing curve was developed: 650℃ and held for 4 hours to initially eliminate forging stress → Cooling down to 300℃ at 40℃ / h and holding for 16 hours to slowly release internal stress and avoid generating new stress due to excessive cooling → Heating up to 890℃ at 60℃ / h and holding for 8 hours to achieve microstructure recrystallization → Air cooling to 400℃ and rapid cooling below the phase transformation point to stabilize the microstructure → Furnace cooling to 300℃ → Heating up to 650℃ at 70℃ / h and holding for 35 hours to deeply eliminate internal stress → Cooling down to 400℃ at 40℃ / h → Cooling down to 150℃ at 50℃ / h.

[0070] During the annealing process, the internal stress of the forging was detected every 2 hours using an ultrasonic stress meter. When the detected internal stress was >80MPa, the cooling rate was automatically reduced by 10℃ / h. The final internal stress value of the finished forging was 72MPa, which meets the material's requirement for low internal stress.

[0071] 6. Finishing process

[0072] Machining peeling: The annealed forgings are peeled using a CNC lathe with a machining allowance of 4mm to ensure the removal of oxide scale and forging defects from the surface of the forgings; after peeling, the surface roughness Ra of the forgings is reduced to Ra≤1.2μm by local grinding with a grinding wheel.

[0073] Sawing: According to the length requirements of the wind turbine main shaft, the forging material is sawn to 11000mm, and the length deviation after sawing is controlled within +1.5mm.

[0074] 7. Finished product inspection

[0075] Appearance and dimensional inspection: Visual inspection of the forging surface reveals no defects such as cracks, inclusions, or folds; the diameter of the forging is measured using a laser diameter gauge, φ600mm±0.3mm, which meets the dimensional accuracy requirements;

[0076] Non-destructive testing: Ultrasonic testing (UT) shall be performed in accordance with EN10228-3 standard, with a testing level of class 1; Magnetic particle testing (MT) shall be performed in accordance with JB / T4730.4 standard, with no surface or near-surface cracks.

[0077] Microstructure inspection: The microstructure of the forging was observed using a metallographic microscope. The grain size was grade 7, with no abnormal structures. Inclusions were graded according to GB / T10561 standard, with a grade of 0.8.

[0078] Mechanical property testing: Tensile, impact, and fatigue tests were conducted on samples taken from different locations of the forging.

[0079] Tensile properties: tensile strength 880MPa, yield strength 680MPa, elongation 18%, all of which are better than the requirements of GB / T3077-1999 "Alloy Structural Steel" standard;

[0080] Impact performance: -40℃ impact absorption energy Akv≥45J, meeting the impact resistance requirements under low temperature conditions;

[0081] Fatigue performance: 10 7 The fatigue strength under multiple cycles is 520 MPa, ensuring the material's resistance to fatigue failure during long-term operation.

[0082] 8. Downstream compatibility pretreatment

[0083] To meet the subsequent precision forging requirements of the materials, customized pretreatment is performed on the finished forgings:

[0084] Surface shot peening: φ0.8mm steel shot is used, shot peening intensity is 0.25mmA, shot peening time is 15min; after shot peening, a residual compressive stress of -350MPa is formed on the surface of the forging, which can improve fatigue strength;

[0085] Phosphating treatment: Zinc-based phosphating solution is used, phosphating temperature is 60℃, and phosphating time is 15min; after phosphating, an 8μm thick phosphating film is formed on the surface of the forging material, and the adhesion of the phosphating film is ≥5MPa, which can reduce the friction between the die and the forging material during the subsequent precision forging process and reduce die wear.

[0086] Pre-treatment inspection: The surface of the forging material is free from shot peening and overheating, and the phosphate film is free from peeling, meeting the surface quality requirements of precision forging.

[0087] III. Implementation Results Verification

[0088] Slab quality: After flaw detection, the continuous casting round slab has a central porosity of only 0.8% and no defects such as axial cracks or subcutaneous bubbles. The slab scrap rate has been reduced from 5% in the traditional process to 0.8%, which greatly improves the slab qualification rate.

[0089] Forging properties: The finished forging has an internal stress of 72MPa, a grain grade of 7, and a fatigue strength of 520MPa. All properties meet the technical requirements of the material, and the fatigue strength is 26% higher than that of forgings of the same specification produced by traditional processes.

[0090] Production energy consumption: Through waste heat recovery and process optimization, the comprehensive energy consumption per ton of forging material has been reduced from 280 kg of standard coal to 230 kg of standard coal in the traditional process, reducing energy consumption by 17.9%; at the same time, CO2 emissions can be reduced by 1,200 tons per 10,000 tons of forging material, which meets environmental protection requirements.

[0091] Downstream compatibility: When the pretreated forgings are used for precision forging, the wear of the precision forging dies is reduced from 0.3 mm / batch to 0.22 mm / batch, and the die life is extended by 28%.

[0092] Example 2: 316L stainless steel φ700mm nuclear power plant tube blank forging

[0093] From the appendix Figure 1 , 2 As shown, the process steps for producing forgings using large vertical continuous casting round billets are as follows:

[0094] 1. Billet preparation

[0095] EBT electric arc furnace steelmaking: tapping temperature 1600-1630℃, control the amount of slag discharged during tapping.

[0096] VOD: Control the target composition of molten steel: C≤0.03%, Si≤0.75%, Mn≤2.0%, Cr 16.5-17.5%, Ni12.5-13.5%, Mo 2.0-2.5%, S≤0.015%, P≤0.025%.

[0097] LF furnace refining: refining temperature 1580-1600℃, with simultaneous calcium feeding line deoxidation, after treatment [H]≤1.6ppm, [N]≤70ppm.

[0098] Calcium treatment for inclusion modification: Calcium wire is fed in at a speed of 2.5 m / min and a feed rate of 0.25 kg / t steel; hard and brittle inclusions such as Al2O3 and TiO2 in the molten steel are converted into CaO-Al2O3-MgO composite inclusions, and the particle size of the inclusions is controlled to be ≤4 μm.

[0099] Large vertical round billet continuous casting: A two-strand continuous casting machine is used, with a billet diameter of φ1000mm, a billet length of 7500mm, and a single billet weight of 52t; multi-site coordination parameters for continuous casting:

[0100] M-EMS: Current 250A, frequency 3Hz, promotes agitation of molten steel in the crystallizer;

[0101] EMBr: Braking strength 0.2T, dynamically adjusted according to the surface temperature of the billet to suppress slag entrapment;

[0102] F-EMS: Current 200A, frequency 4Hz, alternation period 25s, refines the central grains of the billet;

[0103] Secondary cooling zone: 0.75L / kg steel for sections 1-3, 0.45L / kg steel for sections 4-6, and ≤38℃ temperature difference between the inside and outside of the billet.

[0104] Electroslag feeding: The pre-melted slag composition is CaF2 53%, Al2O3 27%, CaO 20%, the arc power is 580kW, the feeding time is 40min, and the shrinkage cavity at the tail end of the billet is eliminated.

[0105] 2. Hot delivery process

[0106] Insulation devices: nano-insulation cover, 7kW electric heat tracing;

[0107] Four thermocouples are arranged on the surface of the billet to monitor the temperature in real time; after hot delivery, the temperature is 870-927℃ when it enters the furnace, with a temperature drop fluctuation of ≤30℃.

[0108] 3. Heating process

[0109] Heating curve: Increase the temperature to 1000℃ at 60℃ / h and hold for 3h → Increase the temperature to 1220℃ at 50℃ / h and hold for 5h;

[0110] Furnace temperature uniformity ±8℃ ensures consistent core and surface temperatures for stainless steel billets.

[0111] 4. Forging process

[0112] The forging process employs a three-pass gradient pressing technique, using a 3150T hydraulic forging press.

[0113] First pass: φ1000mm→φ880mm, reduction rate 23.8%, forging speed 4mm / s, to eliminate porosity in the center of the billet;

[0114] Intermediate isothermal treatment: Hold at 900℃ for 1.5 hours to refine austenite grains and avoid the decrease in corrosion resistance caused by coarse stainless steel grains;

[0115] Second pass: φ880mm→φ780mm, reduction rate 20.2%, forging speed 3.5mm / s, dense structure;

[0116] Intermediate isothermal treatment: holding at 870℃ for 1.2 hours to stabilize the grain structure;

[0117] Third pass: φ780mm→φ700mm, reduction rate 12.3%, forging speed 3mm / s;

[0118] During the forging process, the minimum surface temperature of the forging should be ≥1190℃ to avoid work hardening of stainless steel during forging in the low-temperature zone.

[0119] 5. Annealing process

[0120] To meet the requirements for intergranular corrosion resistance of 316L stainless steel, a customized annealing curve was developed. The final product forging internal stress test value was 68MPa, which meets the requirements for low internal stress in nuclear power tube blanks.

[0121] 6. Finishing process

[0122] Machining peeling: Machining allowance 3.5mm, removing surface oxide scale and defects; after grinding, surface roughness Ra≤1.0μm;

[0123] Sawing: Length 9000mm, length deviation +1mm, which meets the length accuracy requirements of the tube blank.

[0124] 7. Finished product inspection

[0125] Appearance and dimensions: The surface is free of cracks and inclusions; the diameter measured by a laser diameter gauge is φ700mm±0.2mm.

[0126] Non-destructive testing: UTclass1, MT has no defects;

[0127] Microstructure: Grain grade 8, no δ-ferrite or carbide precipitation; inclusion grade 0.7;

[0128] Corrosion resistance:

[0129] Intergranular corrosion test: After the test, the sample showed no intergranular corrosion cracks, meeting the requirements for intergranular corrosion resistance of nuclear power steel;

[0130] Salt spray test was conducted at 35℃ for 1000 hours using a 5% NaCl solution: no red rust was observed on the sample surface, and the corrosion rate was ≤0.001 mm / year, representing a 25% improvement over the 800-hour salt spray life of 316L forgings produced using traditional processes.

[0131] Mechanical properties: tensile strength 530MPa, yield strength 220MPa, elongation 42%, meeting the requirements of ASTM A312-2022 standard.

[0132] 8. Downstream compatibility pretreatment, specifically for perforated tube blanks

[0133] Inner surface eddy current testing: A high-frequency eddy current tester with a testing frequency of 80kHz is used to test the inner surface of the forging material to a depth of 0.5mm to ensure that there are no defects such as surface cracks and pits.

[0134] Coating with glass lubricant: The glass lubricant consists of 60% SiO2, 20% Na2O, and 20% B2O3. The coating temperature is 200℃, and the coating thickness is 4μm. The lubricant can form a protective film during the piercing process, reducing friction between the rolls and the tube blank.

[0135] Pre-treatment inspection: There are no blind spots on the inner surface, the glass lubricant coating is uniform and without peeling, meeting the requirements for perforation processing.

[0136] The embodiments of the present invention are given for the purposes of illustration and description. Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Any changes, modifications, substitutions and variations made by those skilled in the art to the above embodiments within the scope of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for producing forgings using large vertical continuous casting round billets, characterized in that, Includes the following steps: Step 1: The process involves batching, EBT electric furnace steelmaking, LF furnace refining, VD vacuum degassing, continuous casting of φ500-1200mm large vertical round billets, and electroslag feeding to obtain vertical continuous casting round billets. The refining temperature in the LF furnace is 1560-1590℃, and the holding time is 40-60 minutes. Deoxidation is achieved by feeding 0.1-0.2 kg / t of aluminum wire into the steel, and the [O] in the molten steel is controlled to be ≤20ppm. In the continuous casting process of large vertical round billets, a multi-field coordinated control method is adopted, which includes electromagnetic stirring in the crystallizer, electromagnetic braking in the secondary cooling zone, and alternating electromagnetic stirring at the end. Among them, the M-EMS current is 150-300A and the frequency is 2-5Hz, the EMBr braking intensity is 0.1-0.3T, the F-EMS current is 180-250A, the frequency is 3-4Hz, and the alternation period is 20-40s. The secondary cooling zone adopts zoned water mist cooling, with the cooling water volume of the 1st to 3rd stages being 0.6-0.9L / kg steel and the cooling water volume of the 4th to 6th stages being 0.4-0.6L / kg steel, and the temperature difference between the inside and outside of the billet is controlled to be ≤50℃. Electroslag feeding uses pre-melted slag with a composition of 250-55% CaF2, 25-30% Al2O3, and 15-20% CaO, with an arc power of 500-700kW, feeding until the billet is completely solidified; Step 2: The vertical continuous casting round billet obtained in Step 1 is heat-transferred through a nano-insulation cover and an electric heat tracing compensation insulation device. The nano-insulation cover is made of SiO2 aerogel material with a thermal conductivity ≤0.02W / (m・K) and the electric heat tracing power is 5-10kW. Among them, by arranging 3-5 thermocouples on the surface of the billet to monitor the temperature in real time, and starting electric heating when the temperature drop is >50℃, the temperature of the middle surface of the billet is 870-930℃ when it enters the furnace, and the temperature drop fluctuation is ≤30℃. Step 3: The hot-delivered vertical continuous casting round billet is fed into a segmented regenerative combustion heating furnace, and a heating curve is automatically generated according to the steel grade and diameter of the billet. For alloy structural steel φ1000mm casting billets, the temperature is increased to 1000℃ at 60℃ / h and held for 3h, and then increased to 1220℃ at 50℃ / h and held for 5h. Step 4: The heated vertical continuous casting billet is sent to a 1600T or 3150T forging press for multi-pass gradient forging. The total number of forging passes is 3-5, with the first pass having a reduction rate of 20-25%, the intermediate passes having a reduction rate of 15-20%, and the last pass having a reduction rate of 10-15%. The final forging temperature is 850-950℃. During the forging process, an infrared thermal imager is used to monitor the surface temperature of the forging throughout the process. When the temperature is lower than the lower limit of the forging temperature, the pressing speed is reduced from 5 mm / s to 2 mm / s. Finally, forgings with a diameter of φ250-1000 mm and a length of L≤12000 mm are produced. Step 5: Customize the annealing curve according to the steel grade and size of the forging; In step one, the large vertical round billet continuous casting is a single-strand or two-strand continuous casting, and each billet has a length L≤8000mm and a weight ≤70t; in step four, the forging produced corresponds to alloy structural steel, tool steel or stainless steel.

2. The method for producing forgings using large vertical continuous casting round billets according to claim 1, characterized in that, Also includes: Step 6: After annealing, the forging is machined, peeled, locally ground, and sawed in sequence; the machining allowance for peeling is 3-5mm, the surface roughness Ra of the forging after local grinding is ≤1.6μm, and the length deviation of the forging after sawing is ±2mm. Step 7: Conduct visual inspection, dimensional accuracy inspection, non-destructive testing, microstructure inspection, and mechanical property inspection; Step 8: Perform customized pretreatment based on the downstream processing type of the forging material; When used for precision forging of bars, the surface is sequentially shot-peened and phosphated. The shot peening pellet diameter is 0.5-1 mm, the shot peening intensity is 0.2-0.3 mmA, and the phosphate film thickness after phosphate treatment is 5-10 μm. When used for rolling of wire rods, the surface is sequentially descaled by high-pressure water jet and aged at low temperature. The high-pressure water jet pressure is 20-30 MPa, the low-temperature aging temperature is 500℃, and the holding time is 2 h. When used for piercing of pipes, the inner surface is sequentially subjected to eddy current testing and coated with glass lubricant. The eddy current testing frequency is 50-100 kHz, and the glass lubricant thickness is 3-5 μm.

3. The method for producing forgings using large vertical continuous casting round billets according to claim 1, characterized in that: In step one, during billet preparation, after refining in the LF furnace and before vacuum degassing in the VD furnace, a calcium treatment process for inclusion modification is added; the VD time is 15-25 minutes, and the treated molten steel has [H] ≤ 2 ppm and [N] ≤ 80 ppm. In the calcium treatment process for modifying inclusions, calcium wire with a calcium content ≥98% is fed in at a feeding speed of 2-3 m / min and a feeding amount of 0.2-0.3 kg / t steel. This process converts the hard and brittle Al2O3 inclusions in the molten steel into low-melting-point calcium aluminates, while controlling the inclusion particle size to ≤5 μm.

4. The method for producing forgings using large vertical continuous casting round billets according to claim 1, characterized in that: In step five, for 20Cr2Ni4 steel φ550mm forging, the annealing curve is as follows: hold at 650℃ for 4h → cool down to 300℃ at 40℃ / h and hold for 16h → heat up to 890℃ at 60℃ / h and hold for 8h → air cool to 400℃ → furnace cool to 300℃ → heat up to 650℃ at 70℃ / h and hold for 35h → cool down to 400℃ at 40℃ / h → cool down to 150℃ at 50℃ / h.

5. The method for producing forgings using large vertical continuous casting round billets according to claim 1, characterized in that: In step one, the raw materials for the batching consist of scrap steel and the alloy materials required for the corresponding steel grades.

6. The method for producing forgings using large vertical continuous casting round billets according to claim 2, characterized in that: In step seven, the mechanical property test, the tensile strength of alloy structural steel forgings is ≥800MPa, the yield strength is ≥600MPa, and the elongation is ≥15%; the tensile strength of 316L stainless steel forgings is ≥515MPa, the yield strength is ≥205MPa, and the elongation is ≥40%.

7. The forging produced by the method according to any one of claims 1-6, characterized in that: The forging material is a long forging material with a circular cross-section, a diameter of φ250-1000mm, and a length L≤12000mm; the forging material contains [H]≤2ppm, [N]≤80ppm, inclusion rating ≤1.0, and internal stress ≤80MPa; The steel used for forging is alloy structural steel, tool steel, or stainless steel.

8. The application of the forging material according to claim 7, characterized in that: The forging material can be used directly as steel or as a blank for downstream pressure processing.

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